Device and method for attenuating acoustic signals

ABSTRACT

Illustrative embodiments of the present disclosure are directed to methods and devices for attenuating acoustic signals traveling within bodies. A device includes a body and at least one acoustic attenuator disposed along the body. The attenuator includes a cavity and granular particles disposed within the cavity. A liquid is also disposed within the cavity. A restrictor applies a stress to the granular particles within the cavity.

TECHNICAL FIELD

This invention relates to acoustic signals and, more particularly, to attenuating acoustic signals.

BACKGROUND

In the oil and gas industry, subsurface formations are typically probed by well logging tools to determine formation characteristics which can be used to predict or assess the profitability and producibility of subsequent drilling or production operations. In many cases, acoustic logging tools are used to measure formation acoustic properties, which may be used to derive other characteristics of the formations.

Such acoustic logging tools may include acoustic transmitters for transmitting an acoustic signal into a formation and acoustic receivers for receiving acoustic signals that return from the formation. A common problem encountered with acoustic logging tools is that, in some cases, a component of the acoustic signal travels directly through the tool body from the transmitter to the receiver. This component of the acoustic signal does not provide useful information about the formation and instead creates signal noise that interferes with the useful signals coming from the formation. Although conventional solutions to this problem exist, these solutions fail to sufficiently attenuate a broad range of acoustic frequencies.

SUMMARY

Illustrative embodiments are directed to devices for attenuating acoustic signals. In one embodiment, such a device includes a body and a number of attenuators (e.g., at least one attenuator) that are disposed along the body. An acoustic attenuator includes a cavity. A liquid and a granular medium (e.g., a plurality of granular particles) are disposed within the cavity. The attenuator also includes a restrictor for applying a stress to the granular particles within the cavity.

In some embodiments, a plurality of acoustic attenuators is disposed along the body. Some of the attenuators apply a stress to the granular particles and some other attenuators apply a stress with a different value. In this manner, the acoustic attenuators attenuate different acoustic frequencies.

Illustrative embodiments are also directed to a logging tool for performing acoustic investigations of subsurface geological formations traversed by a borehole. The logging tool includes a tool body extending longitudinally. A number of acoustic transmitters are disposed at a longitudinal location on the body and a number of acoustic receivers are disposed at a different longitudinal location on the body. The acoustic logging tool also includes a number of acoustic attenuators that are located between the acoustic transmitters and acoustic receivers. An attenuator includes a cavity disposed within the tool body. A granular medium and a liquid are disposed within the cavity. The liquid is configured to coat the granular particles within the cavity. The attenuator also includes a restrictor for applying a stress to the granular particles within the cavity.

In some embodiments, a plurality of acoustic attenuators is disposed along the tool body. Some of the attenuators apply a stress to the granular particles and some other attenuators apply stress with a different value. In this manner, the acoustic attenuators attenuate different acoustic frequencies.

Illustrative embodiments are also directed to a method for attenuating an acoustic signal passing through a body. The method includes disposing a granular medium (e.g., a plurality of particles) within a cavity disposed along the body and applying a stress to the granular particles within the cavity. In some embodiments, the stress applied to the granular particles is tuned to attenuate particular frequencies of the acoustic signal.

Illustrative embodiments are also directed to a method for predicting attenuation characteristics of a body that includes attenuators disposed with a granular medium. The method includes determining an effective mass of a granular medium under an applied stress. The effective mass is used to determine an effective density for the body. The slowness for the body is determined using the effective density. The process can be repeated for a plurality of different applied stresses.

BRIEF DESCRIPTION OF THE DRAWINGS

Those skilled in the art should more fully appreciate advantages of various embodiments of the present disclosure from the following “Description of Illustrative Embodiments,” discussed with reference to the drawings summarized immediately below:

FIG. 1 shows a schematic view of a drilling assembly in accordance with one embodiment of the present disclosure;

FIG. 2 shows schematic view of a tool body in accordance with one embodiment of the present disclosure;

FIG. 3 shows a schematic view of an attenuator in accordance with one embodiment of the present disclosure;

FIG. 4 shows a schematic view of an attenuator in accordance with another embodiment of the present disclosure;

FIG. 5 shows a schematic view of an attenuator in accordance with another embodiment of the present disclosure;

FIG. 6 shows an assembled view of the attenuator of FIG. 5;

FIG. 7 shows a schematic view an attenuator in accordance with yet another embodiment of the present disclosure;

FIG. 8 shows an assembled view of the attenuator of FIG. 7;

FIG. 9 shows a schematic view of an attenuator in accordance with another specific embodiment of the present disclosure;

FIG. 10 shows a schematic view of an attenuator in accordance with yet another embodiment of the present disclosure;

FIG. 11 shows a schematic view of an attenuator in accordance with another illustrative embodiment of the present disclosure;

FIG. 12 shows a schematic view of an attenuator that is coupled to a tool body in accordance with one embodiment of the present disclosure;

FIG. 13 shows a plot of attenuation versus frequency for a tool body in accordance with one embodiment of the present disclosure;

FIG. 14 shows a plot of attenuation versus frequency for a tool body in accordance with another embodiment of the present disclosure;

FIG. 15 shows a schematic view of a tool body in accordance with another embodiment of the present disclosure;

FIG. 16 shows a cross-sectional view of the tool body of FIG. 15;

FIG. 17 shows a schematic view of a tool body in accordance with another embodiment of the present disclosure;

FIG. 18 shows a schematic view of a tool body in accordance with yet another embodiment of the present disclosure;

FIG. 19 shows a cross-sectional view of the tool body of FIG. 18;

FIGS. 20, 21, and 22 show arrangements of attenuators in accordance with several embodiments of the present disclosure;

FIG. 23 shows an acoustic tool in accordance with one embodiment of the present disclosure;

FIG. 24 shows a plot of dissipative capacity versus frequency in accordance with one embodiment of the disclosure; and

FIG. 25 shows a plot of attenuation versus frequency in accordance with one embodiment of the present disclosure.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Illustrative embodiments of the invention are directed to methods and devices for attenuating a broad range of acoustic signal frequencies traveling through a tool body. To this end, various embodiments of the invention dispose a liquid and a granular medium within a cavity disposed along the tool body. A stress is then applied to the granular medium. The applied stress can be tuned so that the granular medium attenuates particular frequencies of the acoustic signal. In a specific embodiment, a plurality of cavities is used and each cavity is tuned to attenuate a different range of frequencies. In this manner, some embodiments of the present invention can be tuned to attenuate a broad range of acoustic signal frequencies. Details of various embodiments are discussed below.

FIG. 1 shows a drilling assembly 100 in accordance with one embodiment of the present invention. The drilling assembly 100 includes a drill rig 102 disposed over a borehole 104. The drill rig 102 includes a drill string 106 with a down-hole acoustic logging tool 108. The down-hole acoustic logging tool 108 is used for performing acoustic investigations of subsurface formations 110 traversed by the borehole 104. In one embodiment, the acoustic logging tool 108 is part of a logging-while-drilling tool and is configured to investigate subsurface formations while a drilling operation is performed. In other embodiments, however, the acoustic logging tool 108 is independent of the drilling operation.

FIG. 2 shows a tool body 200 in accordance with one embodiment of the present invention. In the specific embodiment shown in FIG. 2, the tool body 200 is a hollow longitudinally extending body that is configured to be placed in a borehole. The tool body 200 includes an outer surface 202 defined by an outer diameter and an inner surface 204 defined by an inner diameter. In one specific embodiment, the tool body 200 is a drill collar of a drilling tool. In such an embodiment, drilling mud flows down into the borehole through the inner diameter of the tool body 200 and flows up the borehole between the formation and the outer surface 202 of the body. It should be noted that discussion of the relative shapes, dimensions, and sizes of the tool body 200 is illustrative and not intended to limit the scope of the invention.

The tool body 200 also includes acoustic sensors disposed on the tool body (e.g., at least one acoustic transmitter 206 and at least one acoustic receiver 208). In some embodiments, the transmitters 206 and receivers 208 are longitudinally spaced from one another. The tool body 200 also includes an attenuation portion 210 defined by the volume of the tool body 200 between the transmitters 206 and the receivers 208. The attenuation portion 210 includes at least one attenuator 212 disposed along the tool body 200 to attenuate acoustic signals traveling through the tool body. In the embodiment shown in FIG. 2, a plurality of attenuators 212 is disposed around the outer surface 202 of the tool body 200.

Various embodiments of the present invention are not limited to the arrangement of attenuators shown in FIG. 2. For example, the attenuators may be disposed adjacent to one or more acoustic sensors (e.g. behind, around, and under the sensors). Additionally, in other embodiments, the attenuators may be disposed along the tool body above or below the attenuation portion (e.g., below the transmitter and/or above the receiver). Such embodiments may help reduce acoustic signals that travel from the surrounding formation back into the tool body.

FIG. 3 shows a cross-sectional view of an attenuator 300 in accordance with one embodiment of the present invention. The attenuator includes a cavity 302 that is disposed within the tool body 304. The cavity 302 has a cylindrical shape and is disposed within an outer surface 305 of the tool body. In additional or alternative embodiments, the cavity 302 may also be disposed on the inner surface of the tool body. In various other embodiments, the cavity 302 is disposed outside the tool body. Furthermore, the cavity 302 may take various other forms. For example, the cavity 302 may be rectangular or elliptical in shape. Also, the cavity 302 may form a channel that extends longitudinally or radially around the outer or inner surfaces of the 304 tool body.

As shown in FIG. 3, a granular medium 306 (e.g., a plurality of granular particles) is disposed within the cavity 302. In various embodiments, a size of the granular particles 306 is significantly smaller than the wavelengths to be attenuated in the tool body 304. In specific embodiments, the granular particles of the granular medium 306 are in the range of 5 microns to 500 microns. These sizes, however, are only examples. In another example, the granular medium 306 includes particles with sizes up to 5000 microns. The size parameters of the granular particles may vary. The ranges and sizes disclosed herein are not intended to limit the scope of the invention.

Also, in various embodiments of the invention, the granular medium 306 is selected so that the particles are stable at high borehole temperatures. To this end, the granular particles can be formed from a metal material, such as aluminum or tungsten, and/or various other materials, such as silicon, cast iron or tungsten carbide. The granular medium 306 may also be selected so that there is a mixture of different sized particles and/or particle materials within the cavity 302. The illustrative materials disclosed herein are not intended to limit the scope of the invention. The material composition of the granular particles may vary beyond the examples provided herein.

Other particle characteristic that may be considered is the particle shape. For example, in one specific embodiment, the granular medium 306 includes particles that are substantially symmetrical (e.g., spherical) with rounded and/or smooth surfaces. In other embodiments, the particles are unsymmetrical with rough surfaces and/or jagged edges. As explained, the shapes of the granular particles may vary. The illustrative shapes disclosed herein are not intended to limit the scope of the invention.

In the specific example of FIG. 3, the cavity 302 also includes a liquid 308, such as water, oil, drilling liquid, a fluorocarbon lubricant, a polymer and/or a gel. In one specific example, the liquid 308 is a silicone oil, such as polydimethylsiloxane. In various illustrative embodiments, the liquid 308 is a viscous liquid that has a viscosity with a value that is greater than water (e.g., 1 cSt). In further specific embodiments, the liquid 308 has a viscosity in the range of 2 to 1,000,000 cSt at a specific application temperature. For example, in a consumer products application, the liquid 308 may have a viscosity in the range of 2 to 1,000,000 cSt at room temperature. In a borehole application, the liquid 308 may have a viscosity in the range of 2 to 1,000,000 cSt at borehole temperatures (e.g., 100-175)C°. The compositions and ranges disclosed herein are not intended to limit the scope of the invention. The composition and viscosity parameters of the liquid 308 may vary beyond the examples provided herein. For example, in some embodiments, the liquid 308 may have a viscosity that is less than water.

In one illustrative embodiment, the volume of the liquid 308 within the cavity 302 is sufficient so that the liquid coats at least a portion of the granular medium 306. In such an embodiment, the volume of the liquid 308 is selected so that it does not saturate the granular medium 306. For example, in one specific embodiment, 110 grams of tungsten powder is coated with 80 mg of 5000 cSt silicone oil. To this end, in various embodiments, a volume ratio within the cavity 302 of the liquid 308 to the granular medium 306 is in the range of 0.001% to 5%. In other embodiments of the invention, the granular medium 306 is saturated by the liquid 308. The ratios and ranges disclosed herein are examples and are not intended to limit the scope of the invention. Liquid volume and granular medium volume parameters may vary beyond the ratios and ranges described herein.

As shown in FIG. 3, the attenuator 300 also includes a restrictor 310 that is configured to apply a stress to the granular medium 306. The inventors recognized that, by modulating the stress applied to the granular medium 306, the attenuator 300 can be tuned to attenuate particular frequencies of an acoustic signal.

The stress applied by the restrictor 310 to the granular medium 306 is an effective stress. An effective stress is defined as external stress applied by the restrictor subtracted by pore pressure (e.g., atmospheric pressure). The stress applied by the restrictor 310 to the granular medium is at least greater than 0 Pa. In the specific example shown in FIG. 3, the restrictor 310 includes a threaded male cap 312 that is configured to mate with female threads within an opening 314 of the cavity 302. When the cap 312 is threaded into the opening 314 of the cavity, the cap makes contact with the granular medium 302 and exerts a stress upon the granular medium. The stress applied to the granular medium 302 can be tuned by turning the cap 312 so that it moves in and out of the cavity 302 (e.g., by modulating the displacement of the cap). In various illustrative embodiments, the stress applied to the granular medium 302 is within the range of 1 Pa to 5 MPa. The stress applied to the granular medium 302 may vary beyond this range. Such examples and ranges are not intended to limit the scope of the invention.

FIG. 4 shows an attenuator 400 in accordance with another embodiment of the present invention. The attenuator 400 is similar to the attenuator disclosed in FIG. 3, with the exception that, in FIG. 4, the restrictor 402 includes a compliant medium 404, such as a plug. The compliant medium 404 is disposed between the threaded cap 406 and the granular medium 408. As the threaded cap 406 is turned into the cavity 410, the compliant medium 404 applies a stress to the granular medium 408. Once in place, the compliant medium 404 applies a constant stress to the granular medium 408. The compliant medium 404 can be made from various different types of materials, such as rubber or silicone. The stress applied to the granular medium 408 can be tuned by selecting from one or more compliant mediums that apply different predetermined values of stress (e.g., by using materials with different elastic moduli). The compliant medium 404 can also have a variety of different geometries (e.g., thicknesses). In this manner, the stress applied to the granular medium 408 can be tuned by selecting from one or more different medium geometries (e.g., using a thicker compliant medium 404 to apply more stress).

FIGS. 5-6 show an attenuator 500 in accordance with another embodiment of the present invention. In FIGS. 5-6, the compliant medium 404 includes a piston 502 and a spring 504. The spring 504 is disposed between the threaded cap 506 and the piston 502. As the threaded cap 506 is turned into the cavity 508, the spring 504 is compressed and applies a stress to the piston 502, which, in turn, applies a stress to the granular medium 510. FIG. 6 shows an assembled view of the threaded cap 506, the spring 504 and the piston 502. Once assembled, the spring 504 applies a constant stress to the granular medium 510 through the piston 502. In one specific embodiment, the spring 504 is a helical coil that is formed from a metal material. The piston 502 can also be formed from a metal material. The stress applied to the granular medium 510 can be tuned by selecting from one or more springs that apply different predetermined values of stress (e.g., by using springs with different spring constants).

Illustrative embodiments of restrictors that use compliant media (e.g., springs and rubber materials) advantageously apply constant stress in borehole environments. Often, over time, tool vibrations in borehole environments arrange granular medium in a manner that will reduce the granular medium's total volume and, as a result, reduce the stress applied to the granular medium. Compliant media, such as springs, automatically adjust to account for this reduction of volume and, as a result, continue to apply a constant stress to the granular medium.

FIGS. 7-8 show an attenuator 700 in accordance with yet another embodiment of the present invention. In FIGS. 7-8, the restrictor assembly 702 lacks male and female threads. Instead, this specific embodiment relies on a heating process, such as a welding or brazing process, to secure the cap 704 within the opening 706 of the cavity 708. FIG. 8 shows an assembled view of the cap 704, the spring 710 and the piston 712. The attenuator includes a bond 714 formed between the cap and the cavity 708 by the heat/pressure process.

FIG. 9 shows an attenuator 900 in accordance with another specific embodiment of the present invention. The embodiment of FIG. 9 includes two variations to the embodiment shown in FIGS. 7-8. For example, instead of a heating process, an interference fit is used to secure the cap 902 within the opening 904 of the cavity 906. In other words, the diameter of the cap 902 is slightly greater than the diameter of the opening 904 and a force is applied to secure the cap within the opening. Also, as opposed to flat cavity walls 716, as shown FIGS. 7-8, in the embodiment of FIG. 9, cavity walls 908 include flares 910. The flares 910 increase the surface area of the cavity 906 that is in contact with the granular medium 912 and the liquid 914. Such flares 910 may further facilitate attenuation of acoustic signals. In various embodiments of the present invention, an amplitude and/or a frequency of the flares 910 is reduced to below the dimensions of the granular particles. Cavities with such “rough” surfaces may potentially better couple acoustic signals with the granular medium 912.

FIG. 10 shows an attenuator 1000 in accordance with yet another embodiment of the present invention. In the specific embodiment shown, a restrictor 1002 includes a cap 1004 with a protrusion 1006 having a particular thickness. The cap 1002 is secured to an opening 1008 of a cavity 1010 using mechanical fasteners 1012, such as screws or rivets. Also, the restrictor 1002 includes a compliant bag or container (e.g., made from rubber or silicone) 1014 that at least partially contains the granular medium 1016. The compliant bag 1010 is placed within a cavity 1010 and the cap 1004 is secured at an opening 1008 of the cavity 1010. In this manner, the protrusion 1006 applies a stress to the compliant bag 1014, which, in turn, applies a stress to the granular medium 1016. The stress applied to the granular medium 1016 can be tuned by, for example, modulating the thickness of the protrusion 1006, modulating the elastic modulus of the protrusion 1006, and/or modulating the elastic modulus of the compliant bag 1014.

FIG. 11 shows an attenuator 1100 in accordance with another illustrative embodiment of the present invention. In FIG. 11, the protrusion 1006 is replaced with a piston 1102 and a spring 1104. When assembled, the spring 1104 applies a force to the piston 1102, which, in turn applies a stress to a compliant bag 1106 and a granular medium 1108 within the bag. The stress applied to the granular medium 1108 can be tuned by, for example, modulating the spring constant and/or modulating the elastic modulus of the compliant bag.

In FIGS. 3-11, the attenuator is formed integral with a tool body. In other words, a section of the tool body is used to contain the granular medium and the liquid. In various other embodiments, however, the attenuator may be a distinct member that is coupled to the tool body (e.g., coupled to an outer or inner surface of the tool body).

FIG. 12 shows an attenuator 1200 that is coupled to a tool body 1202 in accordance with one embodiment of the present invention. The attenuator 1200 is disposed within a through-hole 1204 in the tool body. The attenuator 1200 includes a restrictor 1206 that defines a cavity 1207. The cavity 1207 contains a liquid 1208 and a granular medium 1210. The restrictor 1206 is configured to apply a stress to the granular medium 1210. In one embodiment, the granular medium 1210 is packed into the restrictor 1206 before the attenuator 1200 is coupled to the tool body 1202. A stress can be applied to the granular medium 1210 by, for example, shrink fitting the restrictor 1206 around the liquid 1208 and granular medium 1210. In another example, at least one wall of the restrictor 1206 is purposely deformed so that the deformation applies a stress to the granular medium 1210. In the specific embodiment of FIG. 12, the attenuator 1200 also includes a retaining ring 1212 and an O-ring seal 1214. The retaining ring 1212 secures the container in place within the through-hole 1204, while the O-ring seal 1214 prevents drilling liquid or other contaminants from flowing through the through-hole 1204.

As explained above, the inventors recognized that the attenuation characteristics of a particular attenuator are a function of the stress applied to the granular medium within the attenuator. In illustrative embodiments of the present invention, the tool body includes a plurality of attenuators with at least some of the attenuators tuned to different stress values so that they attenuate different frequencies of the acoustic signal. In this manner, various embodiments of the present invention can attenuate a broad range of acoustic frequencies traveling through the tool body.

FIG. 13 shows a plot 1300 of attenuation versus frequency for a tool body in accordance with one embodiment of the present invention. In this specific example, the tool body includes a number of attenuators. The attenuators are tuned to different applied stresses according to Table 1. The attenuators have a total volume (e.g., a sum of the volume of each cavity) and Table 1 shows the volume fraction of the total volume that is tuned to each applied stress.

TABLE 1 Volume Fraction (%) 33.9 25.9 21.0 9.5 9.7 Applied Stress (MPa) 0.25-0.5 ≈0.5 0.5-1.0 ≈1.0 <1.5 As shown by curve 1302, the attenuators tuned according to Table 1 effectively attenuate frequencies between 8 and 11 kHz by as much as 40 dB/m. In contrast, curve 1304 shows the attenuation characteristics of a tool body with empty cavities (e.g., no liquid or granular medium).

FIG. 14 shows a plot 1400 of attenuation versus frequency for a tool body in accordance with another embodiment of the present invention. In this case, the tool body includes a number of attenuators that are tuned to different applied stresses according to Table 2. As in Table 1, Table 2 shows the volume fraction of the total volume that is tuned to each applied stress.

TABLE 2 Volume Fraction (%) 45.9 17.2 12.5 9.6 7.7 3.5 3.6 Applied >0.25 >0.25 ≈0.25 0.25-0.5 ≈0.5 0.5-1.0 ≈1.0 Stress (MPa) (but < previous column) As shown by curve 1402, the attenuators tuned according to Table 2 effectively attenuate frequencies between 5 and 14 kHz. Also, in this range, the attenuation is as much as 30 dB/m. Curve 1404 shows the attenuation characteristics of a tool body with empty cavities (e.g., no liquid or granular medium).

FIG. 2 shows an example of a tool body 200 that includes a plurality of attenuators 212 with at least some of the attenuators tuned to different stress values so that they attenuate different frequencies of the acoustic signal. In FIG. 2, the attenuators 212 are arranged to form a plurality of circumferential rows spaced apart at selected longitudinal intervals. In a specific example, each row may comprise one or more attenuators 212 (e.g., 5, 10, or 15) azimuthally arranged at substantially evenly spaced intervals about the longitudinal axis of the tool body 200. The plurality of attenuators 212 can be categorized into a plurality of different sections based on the attenuation characteristics of the attenuators. For example, in a first section 214, the attenuators 212 are tuned to attenuate frequencies above 2-4 kHz. The attenuators 212 in this first section 214 might be tuned so that no stress is applied to the granular medium. In the second section 216, a slight stress is applied to the granular medium and the attenuators 212 are thus tuned to attenuate frequencies between 4 kHz and 6 kHz. In a third section 218, a greater stress is applied to the granular medium and thus the attenuators 212 are tuned to attenuate frequencies between 6 kHz and 8 kHz. And, in a fourth section 220, an even greater stress is used to tune the attenuators 212 to attenuate frequencies above 8 kHz. In this manner, the attenuation portion 210 of the tool body 200 is configured to attenuate a broad range of frequencies of the acoustic signal (e.g., 2-10 kHz).

It should be noted that illustrative embodiments of the invention are not limited to such a “sectional” arrangement of attenuators. For example, in other embodiments, the tuned attenuators are not separated by various sections in accordance with their attenuation characteristics. In other embodiments, the differently tuned attenuators are interposed amongst each other. In yet further embodiments, the differently tuned attenuators are interposed amongst each other at random.

FIGS. 15 and 16 show a tool body 1500 in accordance with another embodiment of the present invention. In this embodiment, the tool body 1500 includes seven attenuators 1502 located between a transmitter 1504 and two receivers 1506. Each attenuator 1502 includes a cavity in the shape of a channel 1508 that extends circumferentially around the tool body 1500. In the specific example of FIGS. 15 and 16, four of the channels 1508 are disposed on an outer surface 1510 of the tool body 1500 and three channels are disposed on an inner surface 1512 of the tool body. In other embodiments, however, the channels 1508 may be disposed on only the inner surface 1512 or on only the outer surface 1510 of the tool body. Additionally or alternatively, in other embodiments, the attenuator 1502 may comprise a separate member that couples to the tool body 1500, such as the embodiment in FIG. 12.

Each of the attenuators 1502 also includes a restrictor 1514 for applying a stress to a granular medium 1518 and a liquid 1516 that are disposed within the channels 1508. In the specific example of FIGS. 15 and 16, the restrictors 1514 include an internal sleeve 1520 and an external sleeve 1522 for securing the liquid 1516 and granular medium 1518 within the channels 1508. In the specific embodiment of FIG. 15, ends of the internal and external sleeves 1516, 1518 include O-rings 1524 to prevent in-flow of debris and liquids. The restrictors 1514 also include a compliant medium (e.g., a plug or a spring/piston assembly) for applying a tunable stress to the granular medium 1516. In accordance with a specific embodiment of the invention, each attenuator 1502 is tuned to a different stress and thus attenuates a different frequency of the acoustic signal. In this manner, the seven attenuators 1502 function together to attenuate a broad range of acoustic signals.

Acoustic logging tools in accordance with the embodiment of FIGS. 15 and 16 may advantageously provide the benefit of an additional attenuation mechanism. In some embodiments, the channels may form an array of periodically spaced grooves within the tool body that attenuate acoustic signals. U.S. Pat. No. 5,852,587 to Kostek et al. discloses a method and apparatus for attenuating signals using an array of periodically spaced grooves within the tool body. This patent is hereby incorporated by reference in its entirety.

FIG. 17 shows a tool body 1700 in accordance with another embodiment of the present invention. The tool body 1700 includes a plurality of attenuators 1702, 1703 located between a transmitter 1704 and two receivers 1706. In the specific example of FIG. 17, a plurality of “cylindrical” attenuators 1702, such as the ones disclosed in FIG. 3, are disposed on an outer surface 1708 of the tool body 1700 and a plurality of attenuators 1703 with channels that circumferentially extend around the tool body, such as the ones disclosed in FIGS. 15 and 16, are disposed on an inner surface 1710 of the tool body. In other embodiments, both types of attenuators 1702, 1703 may be disposed on an outer surface 1708 and/or an inner surface 1710 of the tool body 1700.

FIGS. 18 and 19 show a tool body 1800 in accordance with yet another embodiment of the present invention. The tool body 1800 includes a plurality of attenuators 1802 located between a transmitter 1804 and two receivers 1806. In this embodiment, the attenuators 1802 include slot-shaped cavities 1808 disposed along an outer surface 1810 of a tool body. The slot-shaped cavities 1808 have a width and a length and are disposed around the circumference of the tool body 1800. As shown in FIG. 19, each attenuator 1802 includes a restrictor 1810 with a compliant medium 1812 configured to contain and apply a pressure to a granular medium 1814. With respect to FIG. 18, although the slot-shaped cavities 1808 are shown as being oriented longitudinally (parallel to the axis of the tool body 1800), in other embodiments, the slot-shaped cavities 1808 may be oriented axially, radially, tangentially, and/or obliquely.

FIGS. 20, 21, and 22 show arrangements of attenuators in accordance with several embodiments of the present invention. An upper schematic in each figure represents a perspective view of a tool body and a lower schematic represents a lateral cross sectional view of the tool body.

In the example shown in FIG. 20, a tool body 2000 comprises a plurality of cylindrical-shaped cavities 2002 disposed on an outer surface 2004 of the tool body. The cylindrical-shaped cavities 2002 are arranged in a plurality of azimuthally continuous rows. The rows are spaced apart at selected intervals along the central axis of the tool body 2000.

In the example shown in FIG. 21, a tool body 2100 comprises a plurality of slot-shaped cavities 2102 disposed in an outer surface 2104 of the tool body. The cavities 2102 are arranged in a plurality of azimuthally continuous rows spaced at selected intervals with respect to the central axis of the tool body 2100.

In the example shown in FIG. 22, a tool body 2200 comprises a plurality of cylindrically-shaped cavities 2202 arranged in a plurality of azimuthally arranged rows. The rows are spaced apart at various intervals along the central axis of the tool body 2200. The cavities 2202 on each row have openings azimuthally arranged at evenly spaced intervals. The cavities 2202 are formed so that they extend laterally through the tool body 2200 in a direction that is generally tangential with an outer surface 2204 of the tool body.

The present invention is not limited to the specific parameters described herein. Parameters such as location, orientation and size of the one or more cavities of the attenuator may vary. Other examples of attenuator arrangements are disclosed in U.S. Pat. No. 6,643,221 to Hsu et al., which is hereby incorporated by reference in its entirety.

FIG. 23 shows an acoustic tool 2300 in accordance with one embodiment of the present invention. In one specific embodiment, the tool is used as a backing plate 2300 for a series of acoustic transducers 2302. The backing plate 2300 includes two halves that are configured to clamp onto a borehole tool and insulate the acoustic transducers 2302 from acoustic signals traveling through the borehole tool.

The backing plate 2300 includes a tool body having a cylindrical shape with an inner surface 2304 and an outer surface 2306. The series of acoustic sensors 2302 (e.g., transmitters and/or receivers) is mounted on the outer surface 2306 of the backing plate 2300. The cylindrical tool body includes two halves of a cylindrical metal frame 2308, 2310. Each half of the cylindrical member 2308, 2310 includes a plurality of attenuators 2312. The attenuators 2312 include cavities which extend longitudinally from a top surface 2314 of the cylindrical tool body and between the inner surface 2304 and the outer surface 2306 of the tool body. The cavities are arranged at circumferentially spaced apart locations and may extend into the wall of the cylindrical member at various depths. A granular medium and a liquid are disposed within each of the cavities to form the attenuators 2312. The cavities are then sealed at one or both ends, depending on the depth of the holes, using any of the above described restrictors. The restrictors retain the liquid and the granular medium within the cavities and apply stress to the granular medium. The cylindrical tool body 2300 can be molded in two halves and then mounted on the borehole tool (not shown).

In illustrative embodiments of the present invention, a total volume of the attenuators within the tool body can vary. For example, in some specific embodiments, the total volume of the cavities is 5% to 20% of the total volume of the tool body. In FIG. 2, the total volume of the tool body is defined as the volume between the at least one acoustic transmitter 206 and the at least one acoustic receiver 208 (e.g., the attenuation portion 210). Various other embodiments of the present invention are not limited to a range of 5% to 20%. In many cases, the total volume of the cavities may be increased to 50% or 60% of the total volume of the tool body. Generally, the total volume of the cavities can be increased so long as the tool body maintains its mechanical integrity.

Illustrative embodiments of the present invention are also directed to methods and processes for modeling attenuation characteristics of the above described attenuators and tool bodies. To this end, an effective mass M(w) of a granular medium (e.g., a plurality of particles) under an applied stress is determined. The effective mass can be measured using a mechanical shaker and a cup mounted to the mechanical shaker. The cup is filled with the granular medium and a particular stress is applied to the granular medium within the cup. The mechanical shaker oscillates the cup and the granular medium at a series of frequencies. The measurements can be repeated for a number of different applied stresses. The effective mass of the granular media, at a particular stress, can be determined according to the following equation:

$\begin{matrix} {{{M(\omega)} = {\frac{F_{C{(\omega)}}}{a_{C{(\omega)}}} - M_{C}}},} & (1) \end{matrix}$

where M_(C) is the mass of the cup, F_(C)(ω) is an output from a force gauge coupled between the cup and the mechanical shaker, and a_(C)(ω) is an output from an accelerometer coupled to the cup.

Further details on measuring effective mass of a granular media can be found in a publication entitled “Dynamic Effective Mass of Granular Media and the Attenuation of Structure-Borne Sound” by Valenza et al., Phys. Rev. E 80 (2009) 051304, and also in a publication entitled “Effect of Granular Media on the Vibrational Response of a Resonant Structure” by Valenza et al., J. Acoust. Soc. Am. 128, 2768 (2010).

The effective mass of the granular medium includes a real portion M₁ and an imaginary portion M₂. M₂ is the dissipative capacity of the granular media (e.g., M₂=Im[M(ω)]). Dissipative capacity is a measure of the granular medium's ability to dissipate acoustic signals. The inventors recognized that this dissipative capacity can be maximized over a particular acoustic frequency by tuning the stress applied to the granular medium.

FIG. 24 shows a plot 2400 of dissipative capacity versus frequency in accordance with one embodiment of the invention. The plot includes five curves that each show dissipative capacity M₂ characteristics for an attenuator tuned to five different stresses ranging between 0.1 MPa and 2 MPa. The five curves are designated in order of increasing applied stress: 2402, 2404, 2406, 2408 and 2410. The five curves show that a peak in dissipative capacity M₂ can be shifted through a broad frequency range (e.g., 5-12 kHz) by modulating the stress applied to the granular medium.

Using the effective mass of the granular medium at a particular stress, attenuation characteristics for the granular medium within a body (e.g., a drill collar) can be determined. The following equation may be used to determine an effective density ρ_(s) of a body that includes an attenuator with a granular medium under stress:

$\begin{matrix} {{{\rho (\omega)} = {{\left( {1 - \varphi} \right)\rho_{s}} + \frac{\varphi \; {M(\omega)}}{V_{c}}}},} & (2) \end{matrix}$

where ρ_(s) is the density of the body (e.g., density of steel), V_(c) is the volume of a single cavity within the body, and φ is the porosity of the body.

Slowness S(ω) values for the body can be determined using standard acoustic borehole techniques, where the density is given by equation 2 and an elastic constant of the body can be measured independently. The slowness values S(w) of interest are complex slowness values. A complex slowness value S(ω) will include a real portion S₁(ω) and an imaginary portion S₂(ω). Attenuation is related to the imaginary portion of slowness and can be defined as the product of frequency ω and the imaginary portion of slowness S₂(ω) (e.g., ω×S₂(ω). This value can then be converted to a decibel scale.

FIG. 25 shows a plot 2500 of attenuation versus frequency in accordance with one embodiment of the present invention. The plot shows the attenuation characteristics for the attenuator of FIG. 24. In this case, the same five stresses (ranging between 0.1 MPa and 2 MPa) were applied to the granular medium within the attenuator. The five curves are designated in order of increasing applied stress: 2502, 2504, 2506, 2508 and 2510. In contrast, curve 2512 shows the attenuation characteristics of an empty cavity (e.g., no liquid or granular medium). As the stresses applied to the granular medium change, so do the attenuation characteristics of the attenuator. For example, the least stressed granular medium 2502 attenuates approximately 10 decibels from frequencies in the range of 6 to 7 kHz, while the most stressed granular medium 2510 effectively attenuates at least 100 decibels from frequencies in the range of 12 to 14 kHz.

Illustrative embodiments of the present invention are not limited to oil and gas field applications. Various embodiments of the present invention can be used in a broad range of acoustic applications (e.g., telemetry, room acoustics, architectural acoustics, consumer electronics and audio equipment). Furthermore, although some of the embodiments disclosed above describe tools that use attenuators in conjunction with acoustic sensors, other illustrative embodiments of the present invention are directed to applications without acoustic sensors. For example, various embodiments of the present invention are directed to damping and/or insulation materials that incorporate attenuators. In many cases, such damping and insulation materials do not use acoustic sensors.

Although the above discussion discloses various exemplary embodiments of the invention, it should be apparent that those skilled in the art can make various modifications that will achieve some of the advantages of the invention without departing from the true scope of the invention. 

What is claimed is:
 1. A device comprising: a body; at least one acoustic attenuator disposed along the body, the attenuator comprising: a cavity; a plurality of granular particles disposed within the cavity; a liquid disposed within the cavity; and a restrictor configured to apply a stress to the granular particles within the cavity.
 2. The device of claim 1, further comprising: a plurality of acoustic attenuators, wherein at least one attenuator applies a first stress to the granular particles and at least one other attenuator applies a second stress to the granular particles, the first stress and second stress having different values.
 3. The device of claim 1, wherein the restrictor is configured to apply a stress to the granular particles, within the cavity, in the range of 1 Pa to 5 MPa.
 4. The device of claim 1, wherein the liquid has a viscosity in the range of 2 to 1,000,000 cSt at room temperature.
 5. The device of claim 1, wherein a volume ratio within the cavity of the liquid to the granular particles is in the range of 0.001% to 5%.
 6. The device of claim 5, wherein a volume of the liquid in the cavity is configured so that the liquid coats the granular particles.
 7. The device of claim 1, wherein a particle size for the granular particles is in the range of 1 to 500 microns.
 8. The device of claim 1, further comprising: at least one acoustic transmitter disposed on the body; and at least one acoustic receiver disposed on the body; wherein the at least one acoustic attenuator is located between the at least one acoustic receiver and the at least one acoustic transmitter.
 9. The device of claim 8, further comprising a plurality of attenuators located between the at least one acoustic transmitter and the at least one acoustic receiver, the plurality of attenuators comprising a volume between 5% and 20% of a volume of the tool body between the at least one acoustic transmitter and the at least one acoustic receiver.
 10. The device of claim 1, wherein the device is a logging-while-drilling tool and the body is a drill collar.
 11. A logging tool for performing acoustic investigations of subsurface geological formations traversed by a borehole, the logging tool comprising: a tool body extending longitudinally; at least one acoustic transmitter disposed at a first longitudinal location on the body; at least one acoustic receiver disposed at a second longitudinal location on the body; at least one acoustic attenuator located between the at least one acoustic transmitter and the at least one acoustic receiver, the attenuator comprising: a cavity within the tool body; a plurality of granular particles disposed within the cavity; a liquid disposed within the cavity and configured to coat the granular particles within the cavity; and a restrictor configured to apply a stress to the granular particles within the cavity.
 12. The logging tool of claim 11, wherein the restrictor includes a spring configured to apply a stress to the granular particles within the cavity.
 13. The logging tool of claim 11, further comprising: a plurality of attenuators located between the at least one acoustic transmitter and the granular particles and at least one other attenuator applies a second stress to the granular particles, the first stress and second stress having different values.
 14. The logging tool of claim 11, wherein the liquid has a viscosity in the range of 2 to 1,000,000 cSt at borehole temperatures.
 15. The logging tool of claim 11, wherein the restrictor is configured to apply a stress to the granular particles within the cavity in the range of 1 Pa to 5 MPa.
 16. The logging tool of claim 11, wherein a particle size for the granular particles is in the range of 1 to 500 microns.
 17. The logging tool of claim 11, further comprising: a plurality of attenuators located between the at least one acoustic transmitter and the at least one acoustic receiver, the plurality of attenuators comprising a volume between 5% and 20% of a volume of the tool body between the at least one acoustic transmitter and the at least one acoustic receiver.
 18. The logging tool of claim 11, wherein the logging tool is a logging-while-drilling tool and the tool body is a drill collar.
 19. A method for attenuating an acoustic signal passing through a body, the method comprising: disposing a plurality of granular particles within a cavity disposed along the body; and applying a stress to the granular particles within the cavity.
 20. The method of claim 19, wherein the stress applied to the granular particles is tuned to attenuate particular frequencies of the acoustic signal. 